Polymer composition comprising a polyolefin produced in a high pressure process, a high pressure process and an article

ABSTRACT

A polymer composition with improved DC electrical properties, a method for producing the polymer composition, and a cable surrounded by at least one layer comprising the polymer composition are provided.

FIELD OF INVENTION

The invention relates to a polymer composition comprising a polyolefin,a process for producing a polyolefin in a high pressure process and toan article, preferably a cable for wire or cable (W & C) applications,produced using the polymer composition, as well as the use of thepolyolefin in power cable layer.

BACKGROUND ART

Polyolefins produced in a high pressure (HP) process are widely used indemanding polymer applications wherein the polymers must meet highmechanical and/or electrical requirements. For instance in power cableapplications, particularly in medium voltage (MV) and especially in highvoltage (HV) and extra high voltage (EHV) cable applications theelectrical properties of the polymer composition has a significantimportance. Furthermore, the electrical properties may differ indifferent cable applications, as is the case between alternating current(AC) and direct current (DC) cable applications.

A typical power cable comprises a conductor surrounded, at least, by aninner semiconductive layer, an insulation layer and an outersemiconductive layer, in that order.

Space Charge

There is a fundamental difference between AC and DC with respect toelectrical field distribution in the cable. The electric field in an ACcable is easily calculated since it depends on one material propertyonly, namely the relative permittivity (the dielectric constant) with aknown temperature dependence. The electric field will not influence thedielectric constant. On the other hand, the electric field in a DC cableis much more complex and depends on the conduction, trapping andbuild-up of electric charges, so called space charges, inside theinsulation. Space charges inside the insulation will distort theelectric field and may lead to points of very high electric stress,possibly that high that a dielectric failure will follow. Preferablythere should be no space charges present as it will make it possible toeasily design the cable as the electric field distribution in theinsulation will be known.

Normally space charges are located close to the electrodes; charges ofthe same polarity as the nearby electrode are called homocharges,charges of opposite polarity are called heterocharges. The heterochargeswill increase the electric field at this electrode, homocharges willinstead reduce the electric field.

Electrical Conductivity

The DC electrical conductivity is an important material property e.g.for insulating materials for HV DC cables. First of all, the strongtemperature and electric field dependence of this property willinfluence the electric field distribution via space charge build-up asdescribed above. The second issue is the fact that heat will begenerated inside the insulation by the electric leakage current flowingbetween the inner and outer semiconductive layers. This leakage currentdepends on the electric field and the electric conductivity of theinsulation. High conductivity of the insulating material can even leadto thermal runaway under high stress/high temperature conditions. Theconductivity must therefore be kept as low as possible to avoid thermalrunaway.

Compressor Lubricants

HP process is typically operated at high pressures up to 4000 bar. Inknown HP reactor systems the starting monomer(s) need to be compressed(pressurised) before introduced to the actual high pressurepolymerisation reactor. Compressor lubricants are conventionally used inthe hyper-compressor(s) for cylinder lubrication to enable themechanically demanding compression step of starting monomer(s). It iswell known that small amounts of the lubricant normally leaks throughthe seals into the reactor and mixes with the monomer(s). In consequencethe reaction mixture contains traces (up to hundreds of ppm) of thecompressor lubricant during the actual polymerisation step of themonomer(s). These traces of compressor lubricants can have an effect onthe electrical properties of the final polymer.

As examples of commercial compressor lubricants e.g. polyalkylene glycol(PAG): R—[C_(x)R_(y)H_(z)—O]_(n)—H, wherein R can be H or straight chainor branched hydrocarbyl and x, y, x, n are independent integers that canvary in a known manner, and lubricants based on a mineral oil(by-product in the distillation of petroleum) can be mentioned.Compressor lubricants which are based on mineral oils that meet therequirements set for the white mineral oil in European Directive2002/72/EC, Annex V, for plastics used in food contact, are used e.g.for polymerising polymers especially for the food and pharmaceuticalindustry. Such mineral oil-based lubricants contain usually lubricityadditive(s) and may also contain other type of additive(s), such asantioxidants.

WO2009012041 of Dow discloses that in high pressure polymerisationprocess, wherein compressors are used for pressurising the reactants,i.e. one or more monomer(s), the compressor lubricant may have an effecton the properties of the polymerised polymer. The document describes theuse of a polyol polyether which comprises one or none hydroxylfunctionality as a compressor lubricant for preventing prematurecrosslinking particularly of silane-modified HP polyolefins.WO2009012092 of Dow discloses a composition which comprises a HP (i)polyolefin free of silane functionality and (ii) a hydrophobic polyetherpolyol of PAG type wherein at least 50% of its molecules comprise nomore than a single hydroxyl functionality. The component (ii) appears tooriginate from a compressor lubricant. The composition is i.a. for W & Capplications and is stated to reduce dielectrical losses in MV and HVpower cables, see page 2, paragraph 0006. In both applications it isstated that hydrophilic groups (e.g. hydroxyl groups) present in thecompressor lubricant can result in increased water uptake by the polymerwhich in turn can increase electrical losses or, respectively,pre-mature scorch, when the polymer is used as a cable layer material.The problems are solved by a specific PAG type of lubricant with reducedamount of hydroxyl functionalities.

There is a continuous need in the polymer field to find polymers whichare suitable for demanding polymer applications such as wire and cableapplications with high requirements and stringent regulations.

OBJECTS OF THE INVENTION

One of the objects of the present invention is to provide an alternativepolymer composition which comprises a polyolefin with highlyadvantageous properties for use in a cable layer, preferably in a layerof an alternating current (AC) or direct current (DC) cable, morepreferably in a layer of a DC cable.

A further object of the invention is to provide a process forpolymerising a polyolefin in a high pressure reactor resulting in apolyolefin with highly advantageous properties for use in a cable layer,preferably in a layer of an AC or DC cable, more preferably in a layerof a DC cable.

Another object of the invention is to provide a power cable wherein atleast one layer comprises a polymer composition comprising a polyolefin,which is preferably obtainable by a high pressure process and has highlyadvantageous properties for use in a cable layer, preferably in a layerof an AC or DC cable, more preferably in a layer of a DC cable.

Moreover, the invention provides a method for improving the electricalproperties of a polymer composition comprising a polyolefin, which ispreferably obtainable by a high pressure process.

The invention and further objects thereof are described and defined indetails below.

DESCRIPTION OF THE INVENTION

The present invention provides a polymer composition comprising apolyolefin, wherein the polymer composition has an electric conductivityof 0.50×10⁻¹⁵ S/m or less, when measured according to DC conductivitymethod using a tape sample consisting of the polymer composition asdescribed below under “Determination Methods”.

The polymer composition of the invention is also referred herein as“Polymer composition” or “polymer composition”.

The unexpectedly low electrical conductivity of the Polymer compositionis very advantageous i.a. for power cable applications and highlypreferable for DC cable applications. The invention is particularlyadvantageous for DC power cables.

The polyolefin of the Polymer composition is preferably obtainable by ahigh pressure process.

As well known the high pressure reactor system typically comprises acompression zone for a) compressing one or more starting monomer(s) inone or more compressor(s) which are also known as hyper-compressor(s), apolymerisation zone for b) polymerising the monomer(s) in one or morepolymerisation reactor(s) and a recovery zone for c) separatingunreacted products in one or more separators and for recovering theseparated polymer. Moreover, the recovery zone of the HP reactor systemtypically comprises a mixing and pelletising section, such aspelletising extruder, after the separator(s), for recovering theseparated polymer in form of pellets. The process is described in moredetails below.

It has now surprisingly been found that when in a HP reactor system forcompressing the starting monomer(s) a compressor lubricant comprising amineral oil is used in compressors for cylinder lubrication, then theresulting polyolefin has highly advantageous electrical properties suchas reduced, i.e. low, electrical conductivity which is preferably asdefined above and below. This is unexpected, since mineral oils areconventionally used for producing polymers for medical and foodindustry, wherein health aspects are of concern, and not the reducedconductivity, as required for W & C applications.

Compressor lubricant means herein a lubricant used in compressor(s),i.e. in hypercompressor(s), for cylinder lubrication.

“Reduced” or “low” electrical conductivity as used hereininterchangeably mean that the value obtained from the DC conductivitymethod is low, i.e. reduced.

Accordingly the polyolefin of the Polymer composition is preferablyobtainable by a high pressure process comprising

(a) compressing one or more monomer(s) under pressure in a compressor,using a compressor lubricant for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) the compressor lubricant comprises a mineral oil.

The invention further provides method for reducing, i.e. providing low,electric conductivity of a Polymer composition comprising a polyolefin,wherein the method comprises the step of producing the polyolefin in ahigh pressure process comprising:

(a) compressing one or more monomer(s) under pressure in a compressor,using a compressor lubricant for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) a compressor lubricant is used which comprises amineral oil.

A preferred embodiment of this method the electric conductivity of apolymer composition of a cable, preferably of a direct current (DC)power cable, comprising a conductor which is surrounded at least by aninsulation layer, preferably at least by an inner semiconductive layer,an insulation layer and an outer semiconductive layer, is reduced byusing the polymer composition of the invention for producing at leastthe insulation layer.

The invention further provides an article comprising the Polymercomposition as defined above or below. The preferred article is a powercable, preferably a direct current (DC) power cable.

The invention thus further provides a power cable, preferably a directcurrent (DC) power cable, comprising a conductor surrounded by one ormore layers, wherein at least one of said layer(s) comprises a polymercomposition comprising a polyolefin which is obtainable by a highpressure polymerisation process comprising the steps of:

(a) compressing one or more monomer(s) under pressure in a compressor,using a compressor lubricant for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) the compressor lubricant comprises a mineral oil.

The expressions “obtainable by the process” or “produced by the process”are used herein interchangeably and mean the category “product byprocess”, i.e. that the product has a technical feature which is due tothe preparation process.

The invention also provides, independently from the above first cable, asecond power cable, preferably direct current (DC) power cable,comprising a conductor surrounded by one or more layers, wherein atleast one of said layer(s) comprises a polymer composition comprising apolyolefin, wherein the polymer composition has an electric conductivityof 0.50×10⁻¹⁵ S/m or less, when measured according to DC conductivitymethod using a tape sample consisting of the polymer composition, asdescribed below under “Determination Methods”.

The first power cable and the second cable as defined above each form anindependent invention. The unifying feature common to the first andsecond power cables is the reduced electrical conductivity of thePolymer composition which can be expressed by both means, i.e. by meansof the electrical conductivity method or by means of“product-by-process”.

The first and second power cable inventions are commonly referred hereinas the Power cable. It is evident that the below preferred embodiments,subgroups and properties of the invention are generalisable andindependent, i.e. can be combined in any combinations, and furtherdescribe the Polymer composition, the components thereof, the HPprocess, compressor lubricant and the first and second power cableinventions, i.e. the Power cable.

Also the use of the Polymer composition for producing a power cable,preferably a direct current (DC) power cable, and a process forproducing a power cable, preferably a direct current (DC) power cableare provided independently.

Compressor Lubricant

The compressor lubricant used in the polymerization process forproducing the preferred polyolefin of the Polymer composition comprisesmineral oil which is a known petroleum product.

Mineral oils have a well known meaning and are used i.a. for lubricationin commercial lubricants. “Compressor lubricant comprising a mineraloil” and “mineral oil-based compressor lubricants” are used hereininterchangeably.

Mineral oil can be a synthetic mineral oil which is producedsynthetically or a mineral oil obtainable from crude oil refineryprocesses.

Typically, mineral oil, known also as liquid petroleum, is a by-productin the distillation of petroleum to produce gasoline and other petroleumbased products from crude oil.

The mineral oil of the compressor lubricant of the invention ispreferably a paraffinic oil. Such paraffinic oil is derived frompetroleum based hydrocarbon feedstocks.

Mineral oil is preferably the base oil of the compressor lubricant. Thecompressor lubricant may comprise other components, such as lubricityadditive(s), viscosity builders, antioxidants, other additive(s) or anymixtures thereof, as well known in the art.

More preferably, the compressor lubricant comprises a mineral oil whichis conventionally used as compressor lubricants for producing plastics,e.g. LDPE, for food or medical industry, more preferably the compressorlubricant comprises a mineral oil which is a white oil. Even morepreferably the compressor lubricant comprises white oil as the mineraloil and is suitable for the production of polymers for food or medicalindustry. White oil has a well known meaning Moreover such white oilbased compressor lubricants are well known and commercially available.Even more preferably the white oil meets the requirements for a food ormedical white oil.

As, known, the mineral oil, preferably the white mineral oil of thepreferred compressor lubricant contains paraffinic hydrocarbons.

Even more preferably, of the compressor lubricant meets one or more ofthe below embodiments:

-   -   In one preferable embodiment, the mineral oil, preferably the        white mineral oil, of the compressor lubricant has a viscosity        of at least 8.5×10⁻ ⁶ m²/s at 100° C.;    -   In a second preferable embodiment, the mineral oil, preferably        the white mineral oil, of the compressor lubricant contains 5%        per weight (wt %) or less of hydrocarbons with less than 25        carbon atoms;    -   In a third preferable embodiment, the hydrocarbons of the        mineral oil, preferably of the white mineral oil, of the        compressor lubricant have an average molecular weight (Mw) of        480 or more.

The above “amount of hydrocarbons”, “viscosity” and “Mw” are preferablyin accordance with the above European Directive 2002/72/EC of 6 Aug.2002.

It is preferred that the compressor lubricant is according to each ofthe above three embodiments 1-3.

The most preferred compressor lubricant of the invention meets therequirements given for white mineral oil in European Directive2002/72/EC of 6 Aug. 2002, Annex V, for plastics used in food contact.Directive is published e.g. in L 220/18 EN Official Journal of theEuropean Communities 15.8.2002. Accordingly the mineral oil is mostpreferably a white mineral oil which meets said European Directive2002/72/EC of 6 Aug. 2002, Annex V. Moreover it is preferred that thecompressor lubricant complies with said European Directive 2002/72/EC of6 Aug. 2002.

The compressor lubricant of the invention can be a commerciallyavailable compressor lubricant or can be produced by conventional means,and is preferably a commercial lubricant used in high pressurepolymerisation processes for producing plastics for medical or foodapplications. Non-exhaustive examples of preferable commerciallyavailable compressor lubricants are e.g. Exxcolub R Series compressorlubricant for production of polyethylene used in food contact andsupplied i.a. by ExxonMobil, Shell Corena for producing polyethylene forpharmaceutical use and supplied by Shell, or CL-1000-SONO-EU, suppliedby Sonneborn.

The compressor lubricant contains preferably no polyalkyleneglycol basedcomponents.

It is preferred that any mineral oil present in the Polymer compositionof the invention originates from the compressor lubricant used in theprocess equipment during the polymerisation process of the polyolefin.Accordingly, it is preferred that no mineral oil is added to the Polymercomposition or to the polyolefin after the polymerisation thereof.

Traces of the mineral oil originating from the compressor lubricant andpresent, if any, in the produced polyolefin would typically amount inmaximum of up to 0.4 wt % based on the amount of the polyolefin. Thegiven limit is the absolute maximum based on the calculation of theworst scenario where all the lost compressor lubricant (average leakage)would go to the final polyolefin. Such worst scenario is unlikely andnormally the resulting polyolefin contains clearly lower level of themineral oil.

The compressor lubricant of the invention is used in a conventionalmanner and well known to a skilled person for the lubrication of thecompressor(s) in the compressing step (a) of the invention.

Polyolefin

The following preferable embodiments, properties and subgroups of thepolyolefin component suitable for the Polymer composition aregeneralisable so that they can be used in any order or combination tofurther define the preferable embodiments of the Polymer composition.

The polyolefin of the Polymer composition is preferably produced in ahigh pressure process. The term polyolefin means both an olefinhomopolymer and a copolymer of an olefin with one or more comonomer(s).As well known “comonomer” refers to copolymerisable comonomer units.

More preferably the polyolefin is a polyethylene produced in a highpressure process, more preferably a low density polyethylene (LDPE)polymer selected from LDPE homopolymer or LDPE copolymer of ethylenewith one or more comonomer(s). The meaning of LDPE polymer is well knownand documented in the literature. Although the term LDPE is anabbreviation for low density polyethylene, the term is understood not tolimit the density range, but covers the LDPE-like HP polyethylenes withlow, medium and higher densities. The term LDPE describes anddistinguishes only the nature of HP polyethylene with typical features,such as different branching architecture, compared to the PE produced inthe presence of an olefin polymerisation catalyst.

In case of polyolefin copolymer, preferably LDPE copolymer of ethylene,the one or more comonomer(s) may be selected from non-polar comonomer(s)or polar comonomer(s), or from any mixtures thereof, as well known. Asthe polar comonomer, comonomer(s) containing hydroxyl group(s), alkoxygroup(s), carbonyl group(s), carboxyl group(s), ether group(s) or estergroup(s), or a mixture thereof, can used. More preferably, comonomer(s),if present, containing carboxyl and/or ester group(s) are used as saidpolar comonomer. Still more preferably, the polar comonomer(s) of thepolyolefin, preferably of a LDPE copolymer of ethylene, is selected fromthe groups of acrylate(s), methacrylate(s) or acetate(s), or anymixtures thereof. If present in said polyolefin, preferably in a LDPEcopolymer of ethylene, the polar comonomer(s) is preferably selectedfrom the group of alkyl acrylates, alkyl methacrylates or vinyl acetate,or a mixture thereof. Further preferably, said polar comonomers areselected from C1- to C6-alkyl acrylates, C1- to C6-alkyl methacrylatesor vinyl acetate. Still more preferably, said polyolefin, preferably aLDPE copolymer of ethylene, is a copolymer of ethylene with C1- toC4-alkyl acrylate, such as methyl, ethyl, propyl or butyl acrylate, orvinyl acetate, or any mixture thereof.

Non-polar comonomer means herein comonomer(s) which do not containhydroxyl group(s), alkoxy group(s), carbonyl group(s), carboxylgroup(s), ether group(s) or ester group(s). Preferred non-polarcomonomer(s) are selected from the group comprising, preferablyconsisting of, monounsaturated (═one double bond) comonomer(s),preferably olefins, preferably alpha-olefins, more preferably C₃- toC₁₀-alpha-olefins, such as propylene, 1-butene, 1-hexene,4-methyl-1-pentene, styrene, 1-octene, 1-nonene; polyunsaturated (═morethan one double bond) comonomer(s); a silane group containingcomonomer(s), or any mixtures thereof. Polyunsaturated comonomer(s) arefurther described below under Process.

If the LDPE polymer is a copolymer, it preferably comprises 0.001 to 50wt.-%, more preferably 0.05 to 40 wt.-%, still more preferably less than35 wt.-%, still more preferably less than 30 wt.-%, more preferably lessthan 25 wt.-%, of one or more comonomer(s).

The polyolefin, preferably a LDPE polymer, may optionally have anunsaturation which preferably originates from vinyl groups, vinylidenegroups and trans-vinylene groups. The unsaturation can be provided bypolymerizing monomer, preferably ethylene, in the presence of a chaintransfer agent (CTA), which introduces double bonds, e.g. vinyl groupsto the polymer chain, or in the presence of one or more polyunsaturatedcomonomer(s), as mentioned above, and optionally in the presence of achain transfer agent which introduces e.g. vinyl groups to the polymerchain. The unsaturated polyolefins and the preferable unsaturated LDPEpolymers are well known. The unsaturation level can be influenced by theselected polymerization conditions such as peak temperatures andpressure, as well known in the field.

It is well known that e.g. propylene can be used as a comonomer or as achain transfer agent (CTA), or both, whereby it can contribute to thetotal amount of the C—C double bonds, preferably to the total amount ofthe vinyl groups. Herein, when a compound which can also act ascomonomer, such as propylene, is used as CTA for providing double bonds,then said copolymerisable comonomer is not calculated to the comonomercontent.

Typically, and preferably in W & C applications, the density of theethylene homo- or copolymer as said polyolefin, preferably of a LDPEpolymer, is higher than 0.860 g/cm³. Preferably the density of theethylene homo- or copolymer is not higher than 0.960 g/cm³. The MFR₂(2.16 kg, 190 ° C.) of the ethylene homo- or copolymer as said preferredpolymer is preferably from 0.01 to 50 g/10 min, more preferably is from0.1 to 20 g/10 min, and most preferably is from 0.2 to 10 g/10 min.

The preferred polyolefin of the invention is a LDPE homopolymer or LDPEcopolymer as defined above, which may optionally be unsaturated. If theLDPE homopolymer is unsaturated, then the unsaturation is provided by achain transfer agent (CTA) and/or by polymerization conditions. If theLDPE copolymer is unsaturated, then the unsaturation can be provided byone or more of the following means: by a chain transfer agent (CTA), byone or more polyunsaturated comonomer(s) and/or by polymerizationconditions. In case of LDPE copolymer, it is preferably an unsaturatedLDPE copolymer of ethylene with at least one polyunsaturated comonomer,preferably a diene, and optionally with other comonomer(s), such aspolar comonomer(s) which is preferably selected from acrylate or acetatecomonomer(s); more preferably an unsaturated LDPE copolymer of ethylenewith a polyunsaturated comonomer, preferably a diene.

Process

The high pressure (HP) process is the preferred process for producing apolyolefin of the Polymer composition, preferably a low densitypolyethylene (LDPE) polymer selected from LDPE homopolymer or LDPEcopolymer of ethylene with one or more comonomers.

The invention further provides a process for polymerising a polyolefinin a high pressure process which comprises the steps of:

(a) compressing one or more monomer(s) under pressure in a compressor,wherein a compressor lubricant is used for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone(s),

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) a compressor lubricant comprises a mineral oilincluding the preferable embodiments thereof.

Accordingly, the polyolefin of the invention is preferably produced athigh pressure by free radical initiated polymerisation (referred to ashigh pressure radical polymerization). The preferred polyolefin is LDPEhomopolymer or LDPE copolymer of ethylene with one or more comonomer(s),as defined above. The LDPE polymer obtainable by the process of theinvention preferably provides the advantageous electrical properties asdefined above or below. The high pressure (HP) polymerisation and theadjustment of process conditions for further tailoring the otherproperties of the polyolefin depending on the desired end applicationare well known and described in the literature, and can readily be usedby a skilled nerson.

Compression Step a) of the Process of the Invention:

Monomer, preferably ethylene, with one or more optional comonomer(s), isfed to one or more compressor(s) or intensifiers at compressor zone tocompress the monomer(s) up to the desired polymerization pressure and toenable handling of high amounts of monomer(s) at controlled temperature.Typical compressors, i.e. hyper-compressors, for the process can bepiston compressors or diaphragm compressors. The compressor zone usuallycomprises several compressors that can work in series or in parallel.The compressor lubricant of the invention is used for cylinderlubrication in at least one, preferably in all of thehyper-compressor(s), present in the compressor zone. The compressionstep a) comprises usually 2-7 compression steps, often with intermediatecooling zones. Temperature is typically low, usually in the range ofless than 200° C., preferably of less than 100° C. Any recycled monomer,preferably ethylene, and optional comonomer(s) can be added at feasiblepoints depending on the pressure.

Polymerisation Step b) of the Process:

Preferred high pressure polymerisation is effected at a polymerisationzone which comprises one or more polymerisation reactor(s), preferablyat least a tubular reactor or an autoclave reactor, preferably a tubularreactor. The polymerization reactor(s), preferably a tubular reactor,may comprise one or more reactor zones, wherein different polymerizationconditions may occur and/or adjusted as well known in the HP field. Oneor more reactor zone(s) are provided in a known manner with means forfeeding monomer and optional comonomer(s), as well as with means foradding initiator(s) and/or further components, such as CTA(s).Additionally, the polymerization zone may comprise a preheating sectionwhich is preceding or integrated to the polymerization reactor. In onepreferable HP process the monomer, preferably ethylene, optionallytogether with one or more comonomer(s) is polymerized in a preferabletubular reactor, preferably in the presence of chain transfer agent(s).

Tubular Reactor:

The reaction mixture is fed to the tubular reactor. The tubular reactormay be operated as a single-feed system (also known as front feed),wherein the total monomer flow from the compressor zone is fed to theinlet of the first reaction zone of the reactor. Alternatively thetubular reactor may be a multifeed system, wherein e.g the monomer(s),the optional comonomer(s) or further component(s) (like

CTA(s)) coming from the compression zone, separately or in anycombinations, is/are split to two or more streams and the split feed(s)is introduced to the tubular reactor to the different reaction zonesalong the reactor. For instance 10-90% of the total monomer quantity isfed to the first reaction zone and the other 90-10% of the remainingmonomer quantity is optionally further split and each split is injectedat different locations along the reactor. Also the feed of initiator(s)may be split in two or more streams. Moreover, in a multifeed system thesplit streams of monomer(/comonomer) and/or optional furthercomponent(s), such as CTA, and, respectively, the split streams ofinitiator(s) may have the same or different component(s) orconcentrations of the components, or both.

The single feed system for the monomer and optional comonomer(s) ispreferred in the tubular reactor for producing the polyolefin of theinvention.

First part of the tubular reactor is to adjust the temperature of thefeed of monomer, preferably ethylene, and the optional comonomer(s);usual temperature is below 200° C., such as 100-200° C. Then the radicalinitiator is added. As the radical initiator, any compound or a mixturethereof that decomposes to radicals at an elevated temperature can beused. Usable radical initiators, such as peroxides, are commerciallyavailable. The polymerization reaction is exothermic. There can beseveral radical initiator injections points, e.g. 1-5 points, along thereactor usually provided with separate injection pumps. As alreadymentioned also the monomer, preferably ethylene, and optionalcomonomer(s), is added at front and optionally the monomer feed(s) canbe split for the addition of the monomer and/or optional comonomer(s),at any time of the process, at any zone of the tubular reactor and fromone or more injection point(s), e.g. 1-5 point(s), with or withoutseparate compressors.

Furthermore, one or more CTA(s) are preferably used in thepolymerization process of the Polyolefin. Preferred CTA(s) can beselected from one or more non-polar and one or more polar CTA(s), or anymixtures thereof.

Non-polar CTA, if present, is preferably selected from

i) one or more compound(s) which does not contain a polar group selectedfrom nitrile (CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O), carbonyl,carboxyl, ether or ester group(s), or mixtures thereof. Non-polar CTA ispreferably selected from one or more non-aromatic, straight chainbranched or cyclic hydrocarbyl(s), optionally containing a hetero atomsuch as O, N, S, Si or P. More preferably the non-polar CTA(s) isselected from one or more cyclic alpha-olefin(s) of 5 to 12 carbon orone or more straight or branched chain alpha-olefin(s) of 3 to 12 carbonatoms, more preferably from one or more straight or branched chainalpha-olefin(s) of 3 to 6 carbon atoms. The preferred non-polar CTA ispropylene.

The polar CTA, if present, is preferably selected from

i) one or more compound(s) comprising one or more polar group(s)selected from nitrile (CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O),carbonyl, carboxyl, ether or ester group(s), or mixtures thereof;

ii) one or more aromatic organic compound(s), or

iii) any mixture thereof.

Preferably any such polar CTA(s) have up to 12 carbon atoms, e.g. up to10 carbon atoms preferably up to 8 carbon atoms. A preferred optionincludes a straight chain or branched chain alkane(s) having up to 12carbon atoms (e.g. up to 8 carbon atoms) and having at least one nitrile(CN), sulfide, hydroxyl, alkoxy, aldehyl (HC═O), carbonyl, carboxyl orester group.

More preferably the polar CTA(s), if present, is selected from i) one ormore compound(s) containing one or more hydroxyl, alkoxy, HC═O,carbonyl, carboxyl and ester group(s), or a mixture thereof, morepreferably from one or more alcohol, aldehyde and/or ketone compound(s).The preferred polar CTA(s), if present, is a straight chain or branchedchain alcohol(s), aldehyde(s) or ketone(s) having up to 12 carbon atoms,preferably up to 8 carbon atoms, especially up to 6 carbon atoms, mostpreferably, isopropanol (IPA), methylethylketone (MEK) and/orpropionaldehyde (PA).

The amount of the preferable CTA(s) is not limited and can be tailoredby a skilled person within the limits of the invention depending on thedesired end properties of the final polymer. Accordingly, the preferablechain transfer agent(s) can be added in any injection point of thereactor to the polymer mixture. The addition of one or more CTA(s) canbe effected from one or more injection point(s) at any time during thepolymerization.

In case the polymerization of the polyolefin is carried out in thepresence of a CTA mixture comprising one or more polar CTA(s) as definedabove and one or more non-polar CTA(s) as defined above, then the feedratio by weight % of polar CTA to non-polar CTA is preferably

1 to 99 wt % of polar CTA and

1 to 99 wt % of non-polar CTA, based on the combined amount of the feedof polar CTA and the non-polar CTA into the reactor.

The addition of monomer, comonomer(s) and optional CTA(s) may includeand typically includes fresh and recycled feed(s).

The reactor is continuously cooled e.g. by water or steam. The highesttemperature is called peak temperature and the reaction startingtemperature is called initiation temperature.

Suitable temperatures range up to 400° C., preferably from 80 to 350° C.and pressure from 700 bar, preferably 1000 to 4000 bar, more preferablyfrom 1000 to 3500 bar. Pressure can be measured at least aftercompression stage and/or after the tubular reactor. Temperature can bemeasured at several points during all steps. High temperature and highpressure generally increase output. Using various temperature profilesselected by a person skilled in the art will allow control of structureof polymer chain, i.e. Long Chain Branching and/or Short Chainbranching, density, branching factor, distribution of comonomers, MFR,viscosity, Molecular Weight Distribution etc.

The reactor ends conventionally with a valve a so-called productioncontrol valve.

The valve regulates reactor pressure and depressurizes the reactionmixture from reaction pressure to separation pressure.

Recovering Step c) of the Process:

Separation:

The pressure is typically reduced to approx 100 to 450 bar and thereaction mixture is fed to a separator vessel where most of theunreacted, often gaseous, products are removed from the polymer stream.Unreacted products comprise e.g. monomer or the optional comonomer(s),and most of the unreacted components are recovered. The polymer streamis optionally further separated at lower pressure, typically less than 1bar, in a second separator vessel where more of the unreacted productsare recovered. Normally low molecular compounds, i.e. wax, are removedfrom the gas. The gas is usually cooled and cleaned before recycling.

Recovery of the Separated Polymer:

After the separation the obtained polymer is typically in a form of apolymer melt which is normally mixed and pelletized in a pelletisingsection, such as pelletising extruder, arranged in connection to the HPreactor system. Optionally, additive(s), such as antioxidant(s), can beadded in this mixer in a known manner to result in the Polymercomposition.

Further details of the production of ethylene (co)polymers by highpressure radical polymerization can be found i.a. in the Encyclopedia ofPolymer Science and Engineering, Vol. 6 (1986), pp 383-410 andEncyclopedia of Materials: Science and Technology, 2001 Elsevier ScienceLtd.: “Polyethylene: High-pressure, R. Klimesch, D. Littmann and F.-O.Mähling pp. 7181-7184.

As to polymer properties, e.g. MFR, of the polymerised Polymer,preferably LDPE polymer, the properties can be adjusted by using e.g.chain transfer agent during the polymerisation, or by adjusting reactiontemperature or pressure (which also to a certain extent have aninfluence on the unsaturation level).

When an unsaturated LDPE copolymer of ethylene is prepared, then, aswell known, the C—C double bond content can be adjusted by polymerisingthe ethylene e.g. in the presence of one or more polyunsaturatedcomonomer(s), chain transfer agent(s), process conditions, or anycombinations thereof, e.g. using the desired feed ratio between monomer,preferably ethylene, and polyunsaturated comonomer and/or chain transferagent, depending on the nature and amount of C—C double bonds desiredfor the unsaturated LDPE copolymer. I.a. WO 9308222 describes a highpressure radical polymerisation of ethylene with polyunsaturatedmonomers, such as an α,ω-alkadienes, to increase the unsaturation of anethylene copolymer. The non- reacted double bond(s) thus provides i.a.pendant vinyl groups to the formed polymer chain at the site, where thepolyunsaturated comonomer was incorporated by polymerization. As aresult the unsaturation can be uniformly distributed along the polymerchain in random copolymerisation manner. Also e.g. WO 9635732 describeshigh pressure radical polymerisation of ethylene and a certain type ofpolyunsaturated α,ω-divinylsiloxanes. Moreover, as known, e.g. propylenecan be used as a chain transfer agent to provide said double bonds.

Polymer Composition

The Polymer composition of the invention may contain further componentssuch as polymer component(s) and/or additive(s), preferably additive(s),such as antioxidant(s), free radical generating agent(s), such ascrosslinking agent(s), e.g.

organic peroxide(s), scorch retarder(s) (SR), crosslinking booster(s),stabiliser(s), processing aid(s), flame retardant additive(s), watertree retardant additive(s), acid scavenger(s), inorganic filler(s) andvoltage stabilizer(s), as known in the polymer field.

The Polymer composition comprises preferably conventionally usedadditive(s) for W & C applications, such as one or more antioxidant(s)and optionally one or more scorch retarder(s), preferably at least oneor more antioxidant(s). The used amounts of additives are conventionaland well known to a skilled person, e.g. as already described aboveunder “Description of the invention”.

The Polymer composition of the invention comprises typically at least 50wt %, preferably at least 60 w t%, more preferably at least 70 wt%, morepreferably at least 75 wt %, more preferably from 80 to 100 wt % andmore preferably from 85 to 100 wt %, of the polyolefin based on thetotal weight of the polymer component(s) present in the Polymercomposition. The preferred Polymer composition consists of polyolefin asthe only polymer component. The expression means that the Polymercomposition does not contain further polymer components, but thepolyolefin as the sole polymer component. However, it is to beunderstood herein that the Polymer composition may comprise furthercomponents other than polymer components, such as additive(s) which mayoptionally be added in a mixture with a carrier polymer, i.e. in socalled master batch.

The Polymer composition has an electric conductivity of 0.50×10⁻¹⁵ S/mor less, preferably of 0.48×10⁻¹⁵ S/m or less, 0.40×10⁻¹⁵ S/m or less,preferably 0.38×10⁻¹⁵ S/m or less, when measured according to DCconductivity method using a tape sample consisting of the polymercomposition, as described below under “Determination Methods”. Morepreferably, e.g. in some demanding embodiments, the electricconductivity of the Polymer composition is more preferably of 0.35×10⁻¹⁵S/m or less, preferably 0.30×10⁻¹⁵ S/m or less, or even as low as of0.25×10⁻¹⁵ S/m or less, depending on the end application, when measuredaccording to DC conductivity method using a tape sample consisting ofthe polymer composition, as described below under “DeterminationMethods”.

The Polymer composition preferably consist of the polyolefin, preferablypolyethylene, more preferably LDPE homo or copolymer which mayoptionally be unsaturated, as the sole polymer component.

Preferably, the polyolefin provides the advantageous claimed electricalproperties of the invention to the Polymer composition. Accordingly, thepolyolefin, preferably polyethylene, more preferably LDPE homo orcopolymer which may optionally be unsaturated, of the Polymercomposition has an electric conductivity of 0.50×10⁻¹⁵ S/m or less,preferably of 0.48×10⁻¹⁵ S/m or less, 0.40×10⁻¹⁵ S/m or less, preferablyof 0.38×10⁻¹⁵ S/m or less, when measured according to DC conductivitymethod using a tape sample consisting of the polyolefin polymer, asdescribed below under “Determination Methods”. More preferably, e.g. insome demanding embodiments, the electric conductivity of the polyolefin,preferably polyethylene, more preferably the optionally unsaturated LDPEhomo or copolymer, is more preferably of 0.35×10⁻¹⁵ S/m or less,preferably 0.30×10⁻¹⁵ S/m or less, or even as low as of 0.25×10⁻¹⁵ S/mor less, depending on the end application, when measured according to DCconductivity method using a tape sample consisting of the polyolefinpolymer, as described below under “Determination Methods”.

The lower limit of the electric conductivity of the Polymer composition,or, preferably, of the polyolefin is not limited and can be e.g.0.0001×10⁻¹⁵ S/m or more, such as 0.001×10⁻¹⁵ S/m, when measured from atape sample according to said DC conductivity method.

Moreover, the Polymer composition with advantageous electricalproperties can be crosslinked. Accordingly, one preferred Polymercomposition of the invention is crosslinkable. It is preferably used forcrosslinkable cable applications which are subsequently crosslinked.Crosslinking can be effected i.a. by radical reaction using radiation orfree radical generating agent(s), also called crosslinking agent(s),which both terms are interchangeably used herein. Examples of such freeradical generating agents are peroxides including inorganic and organicperoxide(s). A further well known crosslinking method is crosslinkingvia functional groups, e.g. by hydrolysing hydrolysable silane groups,which are attached (either via copolymerisation or via grafting) topolymer, and subsequently condensing the formed silanol groups using asilanol condensation catalyst.

Crosslinking is preferably effected by free radical generating agent(s)which contain(s) at least one —O—O— bond or at least one —N═N− bond.More preferably, the free radical generating agent is a peroxide,whereby the crosslinking is preferably effected using a well knownperoxide crosslinking technology that is based on free radicalcrosslinking and is well known in the field. The peroxide can be anysuitable peroxide, e.g. such as conventionally used in the field.

End Uses and End Applications of the Invention

The new Polymer composition of the invention is highly useful in widevariety of end applications of polymers. The preferred use of thePolymer composition is in W & C applications, more preferably in one ormore layers of a power cable.

A power cable is defined to be a cable transferring energy operating atany voltage, typically operating at voltages higher than 1 kV. Thevoltage applied to the power cable can be alternating (AC), direct (DC),or transient (impulse). The polymer composition of the invention is verysuitable for power cables, especially for power cables operating atvoltages higher than 6 kV to 36 kV (known as medium voltage (MV) cables)and power cables operating at voltages higher than 36 kV, known as highvoltage (HV) cables and extra high voltage (EHV) cables, which EHVcables operate, as well known, at very high voltages. The terms havewell known meanings and indicate the operating level of such cables. ForHV and EHV DC power cables the operating voltage is defined herein asthe electric voltage between ground and the conductor of the highvoltage cable. HV DC power cable and EHV DC power cable can operate e.g.at voltages of 40 kV or higher, even at voltages of 50 kV or higher. EHVDC power cables operate at very high voltage ranges e.g as high as up to800 kV, however without limiting thereto.

The Polymer composition with advantageous DC conductivity properties isthus highly suitable for direct current (DC) power cables operating atany voltages, preferably at higher than 36 kV, such as HV or EHV DCpower cables, as defined above.

In addition to reduced electrical conductivity, the Polymer compositionhas preferably also very good space charge properties which areadvantageous for power cables, particularly for DC power cables.

The invention further provides the use of the polyolefin of theinvention, which is obtainable by the high pressure (HP) process of theinvention, for producing a power cable, preferably a DC power cable.Also the use of a polyolefin which is obtainable by the HP process ofthe invention in at least one layer, preferably at least in aninsulation layer, of a DC power cable is provided. Naturally thepolyolefin is used in the polymer composition of the invention.

The first independent Power cable, preferably a direct current (DC)power cable, comprises a conductor surrounded by one or more layers,wherein at least one of said layer(s) comprises a polymer compositioncomprising a polyolefin as defined above which is obtainable by a highpressure polymerisation process comprising the steps of:

(a) compressing one or more monomer(s) under pressure in a compressor,wherein a compressor lubricant is used for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) a compressor lubricant comprises a mineral oil asdefined above.

The second independent Power cable, preferably a direct current (DC)power cable, comprises a conductor surrounded by one or more layers,wherein at least one of said layer(s) comprises a polymer compositioncomprising a polyolefin as defined above, wherein the polymercomposition, preferably the polyolefin, has an electric conductivity of0.50×10⁻¹⁵ S/m or less, preferably of 0.48×10⁻¹⁵ S/m or less, preferablyof 0.40×10⁻¹⁵ S/m or less, more preferably 0.38×10⁻¹⁵ S/m or less, whenmeasured according to DC conductivity method using a tape sampleconsisting of the polymer composition, or respectively the polyolefin,as described below under “Determination Methods”. More preferably, thepolymer composition of the power cable has the electric conductivity of0.35×10⁻¹⁵ S/m or less, preferably of 0.30×10⁻¹⁵ S/m or less, or even aslow as of 0.25×10⁻¹⁵ S/m or less, when measured according to DCconductivity method using a tape sample consisting of the polymercomposition, as described below under “Determination Methods”.

The preferred Power cable, preferably a direct current (DC) Power cable,comprises a conductor surrounded by one or more layers, wherein at leastone of said layer(s) comprises a polymer composition comprising apolyolefin, wherein

i) the polymer composition, preferably the polyolefin, has an electricconductivity of 0.50×10⁻¹⁵ S/m or less, preferably of 0.48×10⁻¹⁵ S/m orless, 0.40×10⁻¹⁵ S/m or less, preferably of 0.38×10⁻¹⁵ S/m or less, whenmeasured according to DC conductivity method using a tape sampleconsisting of the polymer composition, or, respectively, of thepolyolefin polymer, as described below under “Determination Methods”,more preferably, the polymer composition, preferably the polyolefin, ofthe power cable has the electric conductivity of 0.35×10⁻¹⁵ S/m or less,preferably of 0.30×10⁻¹⁵ S/m or less, or even as low as of 0.25×10⁻¹⁵S/m or less, when measured according to DC conductivity method using atape sample consisting of the polymer composition, or, respectively, ofthe polyolefin polymer, as described below under “DeterminationMethods”,

and wherein the polyolefin is obtainable by a high pressurepolymerisation process comprising the steps of:

(a) compressing one or more monomer(s) under pressure in a compressor,wherein a compressor lubricant is used for lubrication,

(b) polymerising a monomer optionally together with one or morecomonomer(s) in a polymerisation zone,

(c) separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone,

wherein in step a) a compressor lubricant comprises a mineral oil asdefined above.

The preferred properties and embodiments of the compressor lubricant, HPprocess and components of the Polymer composition of the Power cable areas defined above, below or in claims.

The term “conductor” means herein above and below that the conductorcomprises one or more wires. Moreover, the cable may comprise one ormore such conductors. Preferably the conductor is an electricalconductor and comprises one or more metal wires.

In one preferable embodiment of the Power cable of the invention, the atleast one layer is an insulation layer which comprises said polymercomposition of the invention. It is generally known that insulationlayers have high requirements for electrical properties.

In a preferred embodiment, the Power cable is a DC cable, whichcomprises at least an inner semiconductive layer, insulation layer andan outer semiconductive layer, in that order, wherein at least one ofsaid layers, preferably at least the insulation layer, comprises saidPolymer composition as defined above or in claims.

As well known the cable can optionally comprise further layers, e.g.layers surrounding the insulation layer or, if present, the outersemiconductive layers, such as screen(s), a jacketing layer(s), otherprotective layer(s) or any combinations thereof.

More preferably the Power cable is crosslinkable and is crosslinkedbefore the end use application.

The invention also provides a process for producing a Power cable,preferably a crosslinkable Power cable, more preferably a crosslinkableDC Power cable, comprising steps of applying, preferably by(co)extrusion, one or more layers on a conductor wherein at least onelayer comprises said polymer composition of the invention as defined interms of product-by-process according to the first cable invention or interms of electrical conductivity according to the second cableinvention, preferably in terms of product-by-process and electricalconductivity.

The power cable production process of the invention is preferablycarried out by

-   -   providing the Polymer composition of the invention as defined        above or below in claims,    -   mixing, preferably meltmixing in an extruder, the Polymer        composition optionally together with further component(s), such        as further polymer component(s) and/or additive(s),    -   applying a meltmix of the Polymer composition obtained from the        previous step, preferably by (co)extrusion, on a conductor to        form one or more layers,    -   wherein at least one layer comprises said polymer composition of        the invention, and    -   optionally crosslinking at least the layer comprising said        polymer composition of the invention

Melt mixing means mixing above the melting point of at least the majorpolymer component(s) of the obtained mixture and is typically carriedout in a temperature of at least 10-15° C. above the melting orsoftening point of polymer component(s).

Preferably, said Polymer composition is used in form of powder, grain orpellets when provided to the cable production process. Pellets can be ofany size and shape and can be prepared by any conventional pelletisingmethod using any conventional pelletising device, such as pelletisingextruder.

The polymer composition may thus contain additive(s) such as additive(s)conventionally used in W&C polymer applications. Part or all of theoptional additive(s) can be added e.g. to the polyolefin before theabove preferable pellet formation to obtain the Polymer composition. Asan alternative, part or all of the optional additive(s) can be added tothe Polymer composition after the preferable pelletisation step andoptionally the Polymer composition is then further pelletised before theuse in cable preparation process. Also alternatively, part or all of theoptional additive(s) can be added to the Polymer composition inconnection with the preparation process of a cable thereof. Theadditive(s) may be used in conventional amounts. In the preferredembodiment the polymer composition of the invention is provided to thecable production process in a form of premade pellets.

The processing temperatures and devices are well known in the art, e.g.conventional mixers and extruders, such as single or twins screwextruders, are suitable for the process of the invention.

It is preferred that the meltmix of the Polymer composition obtainedfrom meltmixing step consists of the polyolefin of the invention as thesole polymer component. The optional, and preferable, additive(s) can beadded to Polymer composition as such or as a mixture with a carrierpolymer, i.e. in a form of so-called master batch.

The term “(co)extrusion” means herein that in case of two or morelayers, said layers can be extruded in separate steps, or at least twoor all of said layers can be coextruded in a same extrusion step, aswell known in the art. The term “(co)extrusion” means herein also thatall or part of the layer(s) are formed simultaneously using one or moreextrusion heads. For instance triple extrusion can be used for formingthree cable layers.

More preferably the at least one layer comprising the Polymercomposition of obtained Power cable is crosslinkable and crosslinked bya free radical generating agent. “Crosslinkable” means that the cablelayer is crosslinked before the use in the end application thereof. Incrosslinking reaction of a polymer i.a. interpolymer crosslinks(bridges) are primarily formed.

The free radical generating agent, preferably a peroxide, can be presentin the Polymer composition, e.g. in the pellets, before it is providedto the cable production process or the free radical generating agent canbe added to Polymer composition in connection to the cable productionline. The crosslinking can be carried out in an elevated temperature ina manner known in the art.

The crosslinking step may be carried out in connection with theproduction line of the Power cable as a subsequent step and optionallyin a different equipment following the cable formation equipment,whereafter the crosslinked article is recovered.

Determination Methods

Unless otherwise stated in the description or experimental part thefollowing methods were used for the property determinations.

Wt %: % by weight

Melt Flow Rate

The melt flow rate (MFR) is determined according to ISO 1133 and isindicated in g/10 min. The MFR is an indication of the flowability, andhence the processability, of the polymer. The higher the melt flow rate,the lower the viscosity of the polymer. The MFR is determined at 190° C.for polyethylenes and may be determined at different loadings such as2.16 kg (MFR₂) or 21.6 kg (MFR₂₁).

Density

The density was measured according to ISO 1183-2. The sample preparationwas executed according to ISO 1872-2 Table 3 Q (compression moulding).

Molecular Weight

The Mz, Mw, Mn, and MWD are measured by Gel Permeation Chromatography(GPC) for low molecular weight polymers as known in the field.

Comonomer Contents

a) Quantification of Alpha-Olefin Content in Linear Low DensityPolyethylenes and Low Density Polyethylenes by NMR Spectroscopy:

The comonomer content was determined by quantitative 13C nuclearmagnetic resonance (NMR) spectroscopy after basic assignment (J. RandallJMS—Rev. Macromol. Chem. Phys., C29(2&3), 201-317 (1989)). Experimentalparameters were adjusted to ensure measurement of quantitative spectrafor this specific task.

Specifically solution-state NMR spectroscopy was employed using a BrukerAvanceIII 400 spectrometer. Homogeneous samples were prepared bydissolving approximately 0.200 g of polymer in 2.5 ml ofdeuterated-tetrachloroethene in 10 mm sample tubes utilising a heatblock and rotating tube oven at 140° C. Proton decoupled 13C singlepulse NMR spectra with NOE (powergated) were recorded using thefollowing acquisition parameters: a flip-angle of 90 degrees, 4 dummyscans, 4096 transients an acquisition time of 1.6 s, a spectral width of20 kHz, a temperature of 125° C., a bilevel WALTZ proton decouplingscheme and a relaxation delay of 3.0 s. The resulting FID was processedusing the following processing parameters: zero-filling to 32k datapoints and apodisation using a gaussian window function; automaticzeroth and first order phase correction and automatic baselinecorrection using a fifth order polynomial restricted to the region ofinterest.

Quantities were calculated using simple corrected ratios of the signalintegrals of representative sites based upon methods well known in theart.

b) Comonomer Content of Polar Comonomers in Low Density Polyethylene (1)Polymers Containing >6 wt. % polar comonomer units Comonomer content (wt%) was determined in a known manner based on Fourier transform infraredspectroscopy (FTIR) determination calibrated with quantitative nuclearmagnetic resonance (NMR) spectroscopy. Below is exemplified thedetermination of the polar comonomer content of ethylene ethyl acrylate,ethylene butyl acrylate and ethylene methyl acrylate. Film samples ofthe polymers were prepared for the FTIR measurement: 0.5-0.7 mmthickness was used for ethylene butyl acrylate and ethylene ethylacrylate and 0.10 mm film thickness for ethylene methyl acrylate inamount of >6 wt %. Films were pressed using a Specac film press at 150°C., approximately at 5 tons, 1-2 minutes, and then cooled with coldwater in a not controlled manner. The accurate thickness of the obtainedfilm samples was measured.

After the analysis with FTIR, base lines in absorbance mode were drawnfor the peaks to be analysed. The absorbance peak for the comonomer wasnormalised with the absorbance peak of polyethylene (e.g. the peakheight for butyl acrylate or ethyl acrylate at 3450 cm⁻¹ was dividedwith the peak height of polyethylene at 2020 cm⁻¹). The NMR spectroscopycalibration procedure was undertaken in the conventional manner which iswell documented in the literature, explained below. For thedetermination of the content of methyl acrylate a 0.10 mm thick filmsample was prepared. After the analysis the maximum absorbance for thepeak for the methylacrylate at 3455 cm⁻¹ was subtracted with theabsorbance value for the base line at 2475 cm⁻¹(A_(methylacrylate)−A₂₄₇₅). Then the maximum absorbance peak for thepolyethylene peak at 2660 cm⁻¹ was subtracted with the absorbance valuefor the base line at 2475 cm⁻¹ (A₂₆₆₀−A₂₄₇₅). The ratio between(A_(methylacrylate)−A₂₄₇₅) and (A₂₆₆₀−A₂₄₇₅) was then calculated in theconventional manner which is well documented in the literature.

The weight-% can be converted to mol-% by calculation. It is welldocumented in the literature.

Quantification of Copolymer Content in Polymers by NMR Spectroscopy

The comonomer content was determined by quantitative nuclear magneticresonance (NMR) spectroscopy after basic assignment (e.g. “NMR Spectraof Polymers and Polymer Additives”, A. J. Brandolini and D. D. Hills,2000, Marcel Dekker, Inc. New York). Experimental parameters wereadjusted to ensure measurement of quantitative spectra for this specifictask (e.g “200 and More NMR Experiments: A Practical Course”, S. Bergerand S. Braun, 2004, Wiley-VCH, Weinheim).

Quantities were calculated using simple corrected ratios of the signalintegrals of representative sites in a manner known in the art.

(2) Polymers containing 6 wt. % or less polar comonomer units Comonomercontent (wt. %) was determined in a known manner based on Fouriertransform infrared spectroscopy (FTIR) determination calibrated withquantitative nuclear magnetic resonance (NMR) spectroscopy. Below isexemplified the determination of the polar comonomer content of ethylenebutyl acrylate and ethylene methyl acrylate. For the FT-IR measurement afilm samples of 0.05 to 0.12 mm thickness were prepared as describedabove under method 1). The accurate thickness of the obtained filmsamples was measured.

After the analysis with FT-IR base lines in absorbance mode were drawnfor the peaks to be analysed. The maximum absorbance for the peak forthe comonomer (e.g. for methylacrylate at 1164 cm⁻¹ and butylacrylate at1165 cm⁻¹) was subtracted with the absorbance value for the base line at1850 cm⁻¹ (A_(polar comonomer)−A₁₈₅₀). Then the maximum absorbance peakfor polyethylene peak at 2660 cm⁻¹ was subtracted with the absorbancevalue for the base line at 1850 cm⁻¹ (A₂₆₆₀−A₁₈₅₀). The ratio between(A_(comononer)−A₁₈₅₀) and (A₂₆₆₀−A₁₈₅₀) was then calculated. The NMRspectroscopy calibration procedure was undertaken in the conventionalmanner which is well documented in the literature, as described aboveunder method 1).

The weight-% can be converted to mol-% by calculation. It is welldocumented in the literature.

Electric Conductivity Measurement Procedure on Tape Samples

Tape Sample:

A Collin Teach-Line E 20T extruder is used to make the tape of thickness0.15 mm.

A die specially designed for tape extrusion is used: The opening is 100mm wide and 0.35 mm high.

Set Temperatures:

-   Zon 1: 60° C.-   Zon 2: 115° C.-   Zon 3: 120° C.-   Zon 4: 125° C.-   Zon 5: 125° C. (the die)-   Screw speed: 30 rpm.-   Line speed: 1 m/min.

The tape samples consisted of the test polymer composition, or in thepreferable embodiment of the polyolefin. In the below examples thepolyolefin base resin was used without any additives.

Prior to measurements the tapes were conditioned in 50° C. oven at 1 atmfor 120 hours. Conduction current measurements are performed by athree-terminal cell, in nitrogen at a pressure of 3 bar and temperatureat 20° C. Specimens are tested with gold-coated electrodes obtained bycold sputtering. The low voltage electrode has a diameter of 25 mm(measurement area is thus 490 mm²). A guard electrode is situatedaround, but separated from the low voltage electrode. The high voltageelectrode has a diameter of 50 mm, the same dimension of the externaldiameter of the guard electrode.

A DC voltage (U) equal to target electric stress (E)×measured tapethickness (d) is applied on the high voltage electrode. The currentthrough the tape between the high voltage and the low voltage electrodeis measured with an electrometer. The measurements are terminated whenthe current has reached a steady-state level, normally after 24-48hours. The reported conductivity σ (S/m) is calculated from thesteady-state current (I) by the equation

Σ=I/(A×E)

where A is the cross-section, in this case 490 mm², and E is theelectric stress, in this case 40 kV/mm.

Experimental Part Preparation of Polymers of the Examples of the PresentInvention and the Comparative Example

All polymers were low density polyethylenes produced in a high pressurereactor. The production of inventive and comparative polymers isdescribed below: In the below measurements, such as density, MFR andelectrical properties, of the inventive and comparative examples,polymer composition where used which consisted of the polyolefin baseresin without any additives. As to CTA feeds, e.g. the PA content can begiven as liter/hour or kg/h and converted to either units using adensity of PA of 0.807 kg/liter for the recalculation.

Inventive Composition 1

Ethylene with recycled CTA was liquefied by compression and cooling to apressure of 90 bar and a temperature of −30° C. and split up into to twoequal streams of roughly 15-16 tons/hour each. The CTA (methyl ethylketone (MEK)), air and a commercial peroxide radical initiator dissolvedin a solvent were added to the two liquid ethylene streams in individualamounts. The two mixtures were separately pumped through an array of 4intensifiers to reach pressures of 2200-2300 bar and exit temperaturesof around 40° C. These two streams were respectively fed to the front(zone 1) (50%) and side (zone 2) (50%) of a split-feed two-zone tubularreactor. The inner diameters and lengths of the two reactor zones were32 mm and 215 m for zone 1 and 38 mm and 480 m for zone 2. MEK was addedin amounts of 113 kg/h to the front stream to maintain a MFR₂=1.8 g/10min. The front feed stream was passed through a heating section to reacha temperature sufficient for the exothermal polymerization reaction tostart. The reaction reached peak temperatures of 250° C. and 321° C. inthe first and second zones, respectively. The side feed stream cooledthe reaction to an initiation temperature of the second zone of 167° C.Air and peroxide solution was added to the two streams in enough amountsto reach the target peak temperatures. The reaction mixture wasdepressurized by a product valve, cooled and polymer was separated fromunreacted gas.

Inventive Composition 2

Purified ethylene was liquefied by compression and cooling to a pressureof 90 bar and a temperature of −30° C. and split up into to two equalstreams of roughly 14 tons/hour each. The CTA (methyl ethyl ketone(MEK)), air and a commercial peroxide radical initiator dissolved in asolvent were added to the two liquid ethylene streams in individualamounts. The two mixtures were separately pumped through an array of 4intensifiers to reach pressures of 2300 bar and exit temperatures ofaround 40° C. These two streams were respectively fed to the front(zone 1) (50%) and side (zone 2) (50%) of a split-feed two-zone tubularreactor. The inner diameters and lengths of the two reactor zones were32 mm and 200 m for zone 1 and 38 mm and 400 m for zone 2. MEK was addedin amounts of 189 kg/h to the front stream to maintain a MFR₂=0.74 g/10min. The front feed stream was passed through a heating section to reacha temperature sufficient for the exothermal polymerization reaction tostart. The reaction reached peak temperatures of 245° C. and 324° C. inthe first and second zones, respectively. The side feed stream cooledthe reaction to an initiation temperature of the second zone of 165° C.Air and peroxide solution was added to the two streams in enough amountsto reach the target peak temperatures. The reaction mixture wasdepressurized by a product valve, cooled and polymer was separated fromunreacted gas.

Inventive Composition 3

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2600 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 4.2 kg/hour ofpropion aldehyde (PA, CAS number: 123-38-6) was added together withapproximately 98 kg propylene/hour as chain transfer agents to maintainan MFR of 2.1 g/10 min. Here also 1,7-octadiene was added to the reactorin amount of 25 kg/h. The compressed mixture was heated to 165° C. in apreheating section of a front feed two-zone tubular reactor with aninner diameter of ca 40 mm and a total length of 1200 meters. A mixtureof commercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerisation reaction to reach peaktemperatures of ca 281° C. after which it was cooled to approximately203° C. The subsequent 2^(nd) peak reaction temperature was 274° C. Thereaction mixture was depressurised by a kick valve, cooled and polymerwas separated from unreacted gas.

Inventive Composition 4

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2600 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 150 kg propylene/hourwas added as chain transfer agents to maintain an MFR of 2.3 g/10 min.The compressed mixture was heated to 165° C. in a preheating section ofa front feed three-zone tubular reactor with an inner diameter of ca 40mm and a total length of 1200 meters. A mixture of commerciallyavailable peroxide radical initiators dissolved in isododecane wasinjected just after the preheater in an amount sufficient for theexothermal polymerisation reaction to reach peak temperatures of ca 284°C. after which it was cooled to approximately 208° C. The subsequent2^(nd) and 3^(rd) peak reaction temperatures were 274° C. and 268° C.respectively, with a cooling in between down to 238° C. The reactionmixture was depressurised by a kick valve, cooled and polymer wasseparated from unreacted gas.

Inventive Composition 5

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2600 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 14.3 kg/hour ofpropion aldehyde (PA) was added as chain transfer agent to maintain anMFR of 2.05 g/10 min. The compressed mixture was heated to 165° C. in apreheating section of a front feed three-zone tubular reactor with aninner diameter of ca 40 mm and a total length of 1200 meters. A mixtureof commercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerisation reaction to reach peaktemperatures of ca 305° C. after which it was cooled to approximately208° C. The subsequent 2^(nd) and 3^(rd) peak reaction temperatures were286° C. and 278° C. respectively, with a cooling in between down to 237°C. The reaction mixture was depressurised by a kick valve, cooled andpolymer was separated from unreacted gas.

Inventive Composition 6

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2600 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 2.8 kg/hour ofpropion aldehyde (PA) was added together with approximately 93 kgpropylene/hour as chain transfer agents to maintain an MFR of 1.9 g/10min. The compressed mixture was heated to 165° C. in a preheatingsection of a front feed three-zone tubular reactor with an innerdiameter of ca 40 mm and a total length of 1200 meters. A mixture ofcommercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerisation reaction to reach peaktemperatures of ca 280° C. after which it was cooled to approximately211° C. The subsequent 2^(nd) and 3^(rd) peak reaction temperatures were282° C. and 260 ° C. respectively, with a cooling in between down to215° C. The reaction mixture was depressurised by a kick valve, cooledand polymer was separated from unreacted gas.

Inventive Composition 7

Ethylene with recycled CTA was compressed in a 5-stage precompressor anda 2-stage hyper compressor with intermediate cooling to reach initialreaction pressure of ca 2700 bar. The total compressor throughput was ca30 tons/hour. In the compressor area approximately 3.6 kg/hour ofpropion aldehyde (PA) was added together with approximately 78 kgpropylene/hour as chain transfer agents to maintain an MFR of 1.9 g/10min. Here also 1,7-octadiene was added to the reactor in amount of 30kg/h. The compressed mixture was heated to 159° C. in a preheatingsection of a front feed three-zone tubular reactor with an innerdiameter of ca 40 mm and a total length of 1200 meters. A mixture ofcommercially available peroxide radical initiators dissolved inisododecane was injected just after the preheater in an amountsufficient for the exothermal polymerisation reaction to reach peaktemperatures of ca 285 ° C. after which it was cooled to approximately220° C. The subsequent 2^(nd) and 3^(rd) peak reaction temperatures were275° C. and 267° C. respectively, with a cooling in between down to 240°C. The reaction mixture was depressurised by a kick valve, cooled andpolymer was separated from unreacted gas.

Reference Composition 1

Purified ethylene was liquefied by compression and cooling to a pressureof 90 bar and a temperature of −30° C. and split up into to two equalstreams of roughly 14 tons/hour each. The CTA (nonene), air and acommercial peroxide radical initiator dissolved in a solvent were addedto the two liquid ethylene streams in individual amounts. The twomixtures were separately pumped through an array of 4 intensifiers toreach pressures of 2000-2200 bar and exit temperatures of around 40° C.These two streams were respectively fed to the front (zone 1) (50%) andside (zone 2) (50%) of a split-feed two-zone tubular reactor. The innerdiameters and lengths of the two reactor zones were 32 mm and 200 m forzone 1 and 38 mm and 400 m for zone 2. Nonene was added in amounts of146 kg/h to the front stream to maintain a MFR₂=0.26 g/10 min. The frontfeed stream was passed through a heating section to reach a temperaturesufficient for the exothermal polymerization reaction to start. Thereaction reached peak temperatures of 271° C. and 304° C. in the firstand second zones, respectively. The side feed stream cooled the reactionto an initiation temperature of the second zone of 162° C. Air andperoxide solution was added to the two streams in enough amounts toreach the target peak temperatures. The reaction mixture wasdepressurized by a product valve, cooled and polymer was separated fromunreacted gas.

Reference Composition 2

Purified ethylene was liquefied by compression and cooling to a pressureof 90 bar and a temperature of −30° C. and split up into to two equalstreams of roughly 14 tons/hour each. The CTA (methyl ethyl ketone(MEK)), air and a commercial peroxide radical initiator dissolved in asolvent were added to the two liquid ethylene streams in individualamounts. The two mixtures were separately pumped through an array of 4intensifiers to reach pressures of 2100-2200 bar and exit temperaturesof around 40° C. These two streams were respectively fed to the front(zone 1) (50%) and side (zone 2) (50%) of a split-feed two-zone tubularreactor. The inner diameters and lengths of the two reactor zones were32 mm and 200 m for zone 1 and 38 mm and 400 m for zone 2. MEK was addedin amounts of 180 kg/h to the front stream to maintain a MFR₂=0.71 g/10min. The front feed stream was passed through a heating section to reacha temperature sufficient for the exothermal polymerization reaction tostart. The reaction reached peak temperatures of 256° C. and 305° C. inthe first and second zones, respectively. The side feed stream cooledthe reaction to an initiation temperature of the second zone of 168° C.Air and peroxide solution was added to the two streams in enough amountsto reach the target peak temperatures. The reaction mixture wasdepressurized by a product valve, cooled and polymer was separated fromunreacted gas.

Density

Example Density (kg/m³) Inventive Composition 1 922 InventiveComposition 2 922 Inventive Composition 3 923 Inventive Composition 4920 Inventive Composition 5 923 Inventive Composition 6 922 InventiveComposition 7 922 Reference Composition 1 920 Reference Composition 2923

Data and Results on the Electrical Testing:

DC Conductivity Test

The sample preparation and determination method were carried out asdescribed under “Electric Conductivity measurement procedure on tapesamples”.

Also the compression lubricant (commercially available products) andchain transfer agent (CTA) used for producing the inventive andcomparative LDPEs are identified in table 1. The inventive polymercompositions (Inv Composition) and Reference polymer compositions (REComposition) consisted of the corresponding LDPE prepared in the aboveexamples. No additives were added.

TABLE 1 Electric conductivity results Electric conductivity (10⁻¹⁵S/m) - tape Composition consisting of polyolefin examples no. Compressorlubricant base resin RE Composition 1 Klueber Syntheso D 201N, PAG-basedlubricant supplied by Klueber 1.12 Inv Composition 1 MO 200, Mineraloil-based lubricant supplied by Hansen & Rosenthal 0.20 RE Composition 2Klueber Syntheso D 201N, PAG-based lubricant supplied by Klueber 2.05Inv Composition 2 MO 200, Mineral oil-based lubricant supplied by Hansen& Rosenthal 0.30 Inv Composition 3 Shell Corena E 150, Mineral oil-basedlubricant supplied by Shell 0.15 Inv Composition 4 Shell Corena E150,Mineral oil-based lubricant supplied by Shell 0.35 Inv Composition 5Shell Corena E150, Mineral oil-based lubricant supplied by Shell 0.30Inv Composition 6 Shell Corena E150, Mineral oil-based lubricantsupplied by Shell 0.23 Inv Composition 7 M-RARUS PE KPL 201, Mineral oilsupplied by ExxonMobil 0.092

“Mineral oil-based” means that contains also additives not specified byproduct supplier.

The data in Table 2 demonstrate the importance of type of compressorlubricant on the DC electric conductivity of the polyolefin tapesamples. A very low DC conductivity value gives an improved performance.Excellent low values are achieved with the use of mineral oil-basedcompressor lubricants compared to PAG lubricants in the high pressureprocess.

1. A polymer composition comprising a polyolefin, wherein the polymercomposition has an electric conductivity of 0.50×10⁻¹⁵ S/m or less, whenmeasured according to DC conductivity method using a tape sampleconsisting of the polymer composition as described under “DeterminationMethods”.
 2. The polymer composition according to claim 1, wherein thepolymer composition has an electric conductivity of 0.48×10⁻¹⁵ S/m orless, when measured according to DC conductivity method using a tapesample consisting of the polymer composition, as described under“Determination Methods”.
 3. The polymer composition according to claim1, wherein the polyolefin is a low density polyethylene (LDPE) selectedfrom a LDPE homopolymer or LDPE copolymer of ethylene with one or morecomonomer(s), which LDPE homopolymer or LDPE copolymer of ethylene mayoptionally be unsaturated.
 4. The polymer composition according to claim1, wherein the polyolefin is produced in a high pressure processcomprising: (a) compressing one or more monomer(s) under pressure in acompressor, wherein a compressor lubricant is used for lubrication, (b)polymerising a monomer optionally together with one or more comonomer(s)in a polymerisation zone, and (c) separating the obtained polyolefinfrom the unreacted products and recovering the separated polyolefin in arecovery zone, wherein in a) the compressor lubricant comprises amineral oil.
 5. A process for producing a polyolefin of the polymercomposition according to claim 1, wherein the polyolefin is produced ina high pressure process comprising: (a) compressing one or moremonomer(s) under pressure in a compressor, wherein a compressorlubricant is used for lubrication, (b) polymerising a monomer optionallytogether with one or more comonomer(s) in a polymerisation zone, and (c)separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone, wherein in a)the compressor lubricant comprises a mineral oil.
 6. The processaccording to claim 5, wherein in a) the mineral oil of the compressorlubricant is a white mineral oil.
 7. The process according to claim 5,wherein the polymerisation b) is operated at a pressure up to 4000 barand at a temperature of up to 400° C.
 8. A method for reducing, i.e. forproviding low, electrical conductivity of a polymer compositioncomprising a polyolefin, wherein the method comprises: producing thepolyolefin in a high pressure process said producing comprising: (a)compressing one or more monomer(s) under pressure in a compressor,wherein a compressor lubricant is used for lubrication, (b) polymerisinga monomer optionally together with one or more comonomer(s) in apolymerisation zone, and (c) separating the obtained polyolefin from theunreacted products and recovering the separated polyolefin in a recoveryzone, whereby a compressor lubricant which comprises a mineral oil isused in a) for the lubrication.
 9. The method of claim 8, furthercomprising, producing one or more layers of a direct current (DC) cablefrom the polymer composition, the DC cable comprising a conductorsurrounded at least by an inner semiconductive layer, an insulationlayer and an outer semiconductive layer.
 10. A power cable comprising aconductor surrounded by one one or more layers, wherein at least one ofsaid layer(s), comprises a polymer composition comprising a polyolefinwhich is obtainable by a high pressure polymerisation processcomprising: (a) compressing one or more monomer(s) under pressure in acompressor, wherein a compressor lubricant is used for lubrication, (b)polymerising a monomer optionally together with one or more comonomer(s)in a polymerisation zone, and (c) separating the obtained polyolefinfrom the unreacted products and recovering the separated polyolefin in arecovery zone, wherein in a) the compressor lubricant comprises amineral oil.
 11. The cable of claim 10, wherein in a) the compressorlubricant comprises white oil as the mineral oil and is suitable forproduction of polymers for food or medical industry; and wherein thepolymerisation b) is operated at a pressure up to 4000 bar, and at atemperature of up to 400° C.
 12. The cable of claim 10, wherein thepolymer composition has an electric conductivity of 0.50×10⁻¹⁵ S/m orless, when measured according to DC conductivity method using a tapesample consisting of the polymer composition as described under“Determination Methods”.
 13. A power cable comprising a conductorsurrounded by one or more layers, wherein at least one of said layer(s),comprises a polymer composition according to claim 1 comprising apolyolefin, wherein the polymer composition has an electric conductivityof 0.50×10⁻¹⁵ S/m or less, when measured according to DC conductivitymethod using a tape sample consisting of the polymer composition asdescribed under “Determination Methods”.
 14. The power cable of claim13, wherein the polyolefin is obtainable by a high pressurepolymerisation process comprising: (a) compressing one or moremonomer(s) under pressure in a compressor, wherein a compressorlubricant is used for lubrication, (b) polymerising a monomer optionallytogether with one or more comonomer(s) in a polymerisation zone, and (c)separating the obtained polyolefin from the unreacted products andrecovering the separated polyolefin in a recovery zone, wherein in a) acompressor lubricant comprises a mineral oil.
 15. The power cable ofclaim 14, wherein in a) the compressor lubricant comprises white oil asthe mineral oil and is suitable for production of polymers for food ormedical industry; and wherein the polymerisation b) is operated at apressure up to 4000 bar, and at a temperature of up to 400° C.
 16. Aprocess for producing a power cable according to claim 10, comprisingapplying one or more layer(s) on a conductor wherein at least one layercomprises a polymer composition; an optionally crosslinking at least theobtained layer of the cable, wherein the polymer composition has anelectric conductivity of 0.50×10⁻¹⁵ S/m or less, when measured accordingto DC conductivity method using a tape sample consisting of the polymercomposition as described under “Determination Methods”.
 17. (canceled)